1. Field of the Invention
The present invention relates to a resonant inverter and a switching power source unit that easily lower an operating voltage of a switch element.
2. Description of the Related Art
U.S. Pat. No. 3,461,372 discloses a resonant inverter having a plurality of resonance frequencies. A feature of the resonant inverter is that a plurality of parallel resonance circuits, each having a resonance frequency that is an odd multiple of the driving frequency, are connected between the collector and the output of a switch element, and a plurality of serial resonance circuits, each having a resonance frequency that is an even multiple of the driving frequency, are connected between the collector and the emitter of the switch element. With the resonant inverter configured as above, when a square wave is applied between the base and the emitter of the switch element, the waveform between the collector and the emitter of the switch element assumes a square wave form. This is because the even multiple components of the driving frequency, namely the components of twice, four times, and six times of the driving frequency have low impedance, and therefore the even multiple components higher than eight times having a small energy, and the odd multiple components expressed as fundamental, three times, five times, . . . , are added in the waveform between the collector and the emitter of the switch element (see FIG. 6). In the resonant inverter the three times, five times, and seven times components, which are the odd multiple components of the waveform between the collector and the emitter of the switch element, are blocked by parallel resonance, and therefore an output voltage of a sine wave shape equal to the fundamental frequency having a largest energy can be provided to the output end.
A resonant converter proposed in U.S. Pat. No. 7,889,519 includes, in a resonant inverter unit, an LC serial resonance circuit having a resonance frequency that is twice of a driving frequency and provided between the drain and the source (between the collector and the emitter) of a switch element. With this circuit, a collector-emitter (drain-source) voltage of the switch can be lowered compared with a conventional resonant inverter, by arranging the inverter unit such that drain-source (collector-emitter) impedance of the switch element becomes lowest around the frequency 0 and twice of the driving frequency (see FIG. 7), according to the cited Literature. Thus, the conventional techniques have focused on shaping the switch waveform to realize an ideal action, to thereby improve the operating voltage of the switch element.
With the conventional resonant converter cited above, however, when the LC serial resonance circuit having the resonance frequency that is twice of the driving frequency is provided between the collector and the emitter (the drain and the source) of the switch element in the resonant inverter unit, the operating voltage of the switch element does not always become lowest and hence the life span of the switch may be degraded. The present invention has been accomplished in view of the drawbacks incidental to the conventional technique, and provides a resonant inverter and a switching power source unit that are reliable and do not affect the life span of the switch element.